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1.
Remobilization of authigenic uranium in marine sediments by bioturbation   总被引：1，自引：0，他引：1  
Uranium behaves as a nearly conservative element in oxygenated seawater, but it is precipitated under chemically reducing conditions that occur in sediments underlying low-oxygen bottom water or in sediments receiving high fluxes of particulate organic carbon. Sites characterized by a range of bottom-water oxygen (BWO) and organic carbon flux (OCF) were studied to better understand the conditions that determine formation and preservation of authigenic U in marine sediments. Our study areas are located in the mid latitudes of the northeast Pacific and the northwest Atlantic Oceans, and all sites receive moderate (0.5 g/cm2 kyr) to high (2.8 g/cm2 kyr) OCF to the sediments. BWO concentrations vary substantially among the sites, ranging from <3 to ∼270 μM. A mass balance approach was used to evaluate authigenic U remobilization at each site. Within each region studied, the supply of particulate nonlithogenic U associated with sinking particles was evaluated by means of sediment traps. The diffusive flux of U into sediments was calculated from pore-water U concentration profiles. These combined sources were compared with the burial rate of authigenic U to assess the efficiency of its preservation. A large fraction (one-third to two-thirds) of the authigenic U precipitated in these sediments via diffusion supply is later regenerated, even under very low BWO concentrations (∼15 μM). Bioturbating organisms periodically mix authigenic U-containing sediment upward toward the sediment-water interface, where more oxidizing conditions lead to the remobilization of authigenic U and its loss to bottom waters.  相似文献   

2.
《Geochimica et cosmochimica acta》1999,63(11-12):1735-1750
We analyzed the redox sensitive elements V, Mo, U, Re and Cd in surface sediments from the Northwest African margin, the U.S. Northwest margin and the Arabian Sea to determine their response under a range of redox conditions. Where oxygen penetrates 1 cm or less into the sediments, Mo and V diffuse to the overlying water as Mn is reduced and remobilized. Authigenic enrichments of U, Re and Cd are evident under these redox conditions. With the onset of sulfate reduction, all of the metals accumulate authigenically with Re being by far the most enriched. General trends in authigenic metal accumulation are described by calculating authigenic fluxes for the 3 main redox regimes: oxic, reducing where oxygen penetrates ≤1 cm, and anoxic conditions. Using a simple diagenesis model and global estimates of organic carbon rain rate and bottom water oxygen concentrations, we calculate the area of sediments below 1000 m water depth in which oxygen penetration is ≤1 cm to be 4% of the ocean floor. We conclude that sediments where oxygen penetrates ≤1 cm release Mn, V and Mo to seawater at rates of 140%–260%, 60%–150% and 5%–10% of their respective riverine fluxes, using the authigenic metal concentrations and accumulation rates from this work and other literature. These sediments are sinks for Re, Cd and U, with burial fluxes of 70%–140%, 30%–80% and 20%–40%, respectively, of their dissolved riverine inputs. We modeled the sensitivity of the response of seawater Re, Cd and V concentrations to changes in the area of reducing sediments where oxygen penetrates ≤1 cm. Our analysis suggests a negligible change in seawater Re concentration, whereas seawater concentrations of Cd and V could have decreased and increased, respectively, by 5%–10% over 20 kyr if the area of reducing sediments increased by a factor of 2 and by 10%–20% if the area increased by a factor of 3. The concentration variations for a factor of 2 increase in the area of reducing sediments are at about the level of uncertainty of Cd/Ca and V/Ca ratios observed in foraminifera shells over the last 40 kyr. This implies that the area of reducing sediments in the ocean deeper than 1000 m (4%) has not been greater than twice the present value in the recent past.  相似文献   

3.
The marine chemistry of iodine in anoxic basins   总被引：1，自引：0，他引：1  
The distribution of dissolved iodate and iodide has been determined in two anoxic basins, the Black Sea and the Cariaco Trench; and the oxic Venezuela Basin which serves as a comparison for normal oceanic conditions. In normal oceanic waters, iodate is the predominant species; its concentration is lowest at the surface (ca. 0.3 μM) and increases with depth to ca. 0.5 μM. In contrast, the iodide concentration shows maximum values in surface waters and rapidly decreases to <0.01 μM below the euphotic zone. In anoxic basins, the reduced pE reverses this trend. The concentration of iodide increases rapidly in the oxygen-sulfide mixing zone from 0.02 to 0.46 μM, and 0.01 to 0.43 μM, in the Cariaco Trench and the Black Sea, respectively. The iodate concentration, meanwhile, decreases to zero.The total iodine to salinity ratio is lower in the surface waters with a range of 7.3–12.1 nmoles/g suggesting a possible depletion by organisms. In the anoxic basins, a maximum in this ratio is observed just above the oxygen-sulfide boundary (15–17 nmoles/g) and is indicative of particle dissolution in a strong pycnocline. In the anoxic zone of the Cariaco Trench, the ratio is constant at 12.3 nmoles/g, whereas in the Black Sea, it increases with depth from 10.0 to 19.3 nmoles/g, suggesting a possible flux of iodide from the bottom sediments.By considering the distribution of iodate and iodide in oxic and anoxic basins, the lower limit of the pE of the oxic ocean is estimated to be 10.7, given our present analytical capability. Thermodynamic considerations further suggest that the iodide-iodate couple is a poor indicator for the pE of the oceans with a limited usable range of 10.0–10.7.  相似文献   

4.
Pore water and solid phase data for redox-sensitive metals (Mn, Fe, V, Mo and U) were collected on a transect across the Peru upwelling area (11°S) at water depths between 78 and 2025 m and bottom water oxygen concentrations ranging from ∼0 to 93 μM. By comparing authigenic mass accumulation rates and diffusive benthic fluxes, we evaluate the respective mechanisms of trace metal accumulation, retention and remobilization across the oxygen minimum zone (OMZ) and with respect to oxygen fluctuations in the water column related to the El Niño Southern Oscillation (ENSO).Sediments within the permanent OMZ are characterized by diffusive uptake and authigenic fixation of U, V and Mo as well as diffusive loss of Mn and Fe across the benthic boundary. Some of the dissolved Mn and Fe in the water column re-precipitate at the oxycline and shuttle particle-reactive trace metals to the sediment surface at the lower and upper boundary of the OMZ. At the lower boundary, pore waters are not sufficiently sulfidic as to enable an efficient authigenic V and Mo fixation. As a consequence, sediments below the OMZ are preferentially enriched in U which is delivered via both in situ precipitation and lateral supply of U-rich phosphorites from further upslope. Trace metal cycling on the Peruvian shelf is strongly affected by ENSO-related oxygen fluctuations in bottom water. During periods of shelf oxygenation, surface sediments receive particulate V and Mo with metal (oxyhydr)oxides that derive from both terrigenous sources and precipitation at the retreating oxycline. After the recurrence of anoxic conditions, metal (oxyhydr)oxides are reductively dissolved and the hereby liberated V and Mo are authigenically removed. This alternation between supply of particle-reactive trace metals during oxic periods and fixation during anoxic periods leads to a preferential accumulation of V and Mo compared to U on the Peruvian shelf. The decoupling of V, Mo and U accumulation is further accentuated by the varying susceptibility to re-oxidation of the different authigenic metal phases. While authigenic U and V are readily re-oxidized and recycled during periods of shelf oxygenation, the sequestration of Mo by authigenic pyrite is favored by the transient occurrence of oxidizing conditions.Our findings reveal that redox-sensitive trace metals respond in specific manner to short-term oxygen fluctuations in the water column. The relative enrichment patterns identified might be useful for the reconstruction of past OMZ extension and large-scale redox oscillations in the geological record.  相似文献   

5.
《Applied Geochemistry》1999,14(5):655-667
Sediments sampled at a hydrocarbon-contaminated, glacial-outwash, sandy aquifer near Bemidji, Minnesota, were analyzed for sediment-associated Fe with several techniques. Extraction with 0.5 M HCl dissolved poorly crystalline Fe oxides and small amounts of Fe in crystalline Fe oxides, and extracted Fe from phyllosilicates. Use of Ti-citrate-EDTA-bicarbonate results in more complete removal of crystalline Fe oxides. The average HCl-extractable Fe(III) concentration in the sediments closest to the crude-oil contamination (16.2 μmol/g) has been reduced by up to 30% from background values (23.8 μmol/g) as a result of Fe(III) reduction in contaminated anoxic groundwater. Iron(II) concentrations are elevated in sediments within an anoxic plume in the aquifer. Iron(II) values under the oil body (19.2 μmol/g) are as much as 4 times those in the background sediments (4.6 μmol/g), indicating incorporation of reduced Fe in the contaminated sediments. A 70% increase in total extractable Fe at the anoxic/oxic transition zone indicates reoxidation and precipitation of Fe mobilized from sediment in the anoxic plume. Scanning electron microscopy detected authigenic ferroan calcite in the anoxic sediments and confirmed abundant Fe(III) oxyhydroxides at the anoxic/oxic boundary. The redox biogeochemistry of Fe in this system is coupled to contaminant degradation and is important in predicting processes of hydrocarbon degradation.  相似文献   

6.
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7.
Enrichment of molybdenum (Mo) in reducing sediments due to authigenic fixation in anoxic interstitial waters could provide an indicator of hypoxic conditions that integrates over substantial temporal extents. Sediments maintained under controlled dissolved oxygen (DO) conditions showed elevated concentrations of Mo when exposed to low DO concentrations. Mo accumulation was linearly related to time of exposure in treatments below 2.8 mg O2/L, with less or no accumulation at higher concentrations. Rates of accumulation were independent of DO concentration below 2.8 mg/L. Accumulation occurred at DO concentrations higher than those limiting accumulation in field settings, with rates in the lowest treatments similar to those in sediments of deep basins with restricted circulation and low DO concentrations.  相似文献   

8.
Uranium geochemistry has been investigated in three acid lakes located on the Canadian Shield and one circumneutral lake in the Appalachian Region of Eastern Canada. In all Shield lakes, dissolved U concentrations were higher in the porewater than in the overlying water. In one of them, whose hypolimnion is perennially oxic, U released to porewater at depths of Fe remobilization was removed from the porewater at depths of Fe oxyhydroxides precipitation; these similarities in the U and Fe profiles indicate that part of the U becomes associated to Fe oxyhydroxides. The dissolved U and Fe profiles in the other two Shield lakes, whose hypolimnions were anoxic when sampled, did not show any significant recycling of these elements in the vicinity of the sediment-water interface and both elements diffused from the sediment to the overlying water. In contrast, in the Appalachian Lake, dissolved U concentrations were higher in the overlying water than in porewater, strongly decreased at the vicinity of the sediment-water interface and then remained relatively constant with sediment depth. Diagenetic modeling of the porewater U profiles, assuming steady-state, reveals that authigenic U always represented ?3% of the total U concentration in the sediments of all lakes. This observation indicates that diagenetic reactions involving U are not quantitatively important and that most of the U was delivered to the sediments at our study sites as particulate U and not through diffusion across the sediment-water interface, as is seen in continental margin sediments. Comparison of the U:Corg and U:Fe molar ratios in diagenetic material collected across the sediment-water interface with Teflon sheets and in surface sediments (0-0.5 cm) of the lake having a perennially oxic hypolimnion suggest that solid phase U was mainly bound to organic matter originating from the watershed; a strong statistical correlation between sediment non-lithogenic U and Corg in the Appalachian Lake supports this contention. Thermodynamic calculations of saturation states suggest that dissolved U was not removed from porewater through precipitation of UO2(s), U3O7(s) and U3O8(s) as previously proposed in the literature.  相似文献   

9.
We have measured U in benthic incubation chambers, sediment pore waters, and in sediments along the California continental margin. Sedimentary U uptake rates, based on a combination of sediment pore water profiles and benthic incubation chambers, generally agree with those predicted from sediment accumulation rate data. This agreement supports the view that most of the continental margin sedimentary U is delivered by diffusion across the seawater-sediment boundary. The average rate of authigenic U accumulation for all the sites examined here is ∼−0.2 nmol cm-2 y-1, which is consistent with published global estimates of sedimentary U uptake. In addition, the accumulation rate of U in sediments exhibits a nonlinear relationship with the oxygen penetration depth and a linear relationship with the organic carbon rain rate. These relationships highlight the potential utility for the U accumulation rate as a proxy for these processes.  相似文献   

10.
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11.
We measured the vertical distributions of Mo, Fe, Mn, sulfide, sulfate, organic carbon, major ions, and pH in sediment porewater from one perennially oxic and three seasonally anoxic lacustrine basins in Eastern Canada, as well as those of Mo, acid volatile sulfide, Fe, Mn, Al, organic C, 210Pb and 137Cs in sediment cores from the same sites. The only input of anthropogenic Mo to these lakes comes from atmospheric deposition.The relatively monotonous distribution of Mo in the porewater of three seasonally anoxic basins suggests that Mo is not redistributed in the sedimentary column during periods of anoxia. In contrast, porewater Mo profiles obtained at three sampling dates in a perennially oxic basin display sharp Mo peaks below the sediment-water interface, indicating redistribution subsequent to deposition. Modeling of these latter porewater Mo profiles with a diagenetic reaction-transport equation coupled to comparisons among the various porewater and solid phase profiles reveal that Mo is released at 1-2 cm depth as a consequence of the reductive dissolution of Fe oxyhydroxides and scavenged both at the vicinity of the sediment-water interface, by re-adsorption onto authigenic Fe oxyhydroxides, and deeper in the sediments where dissolved sulfide concentrations are higher. The estimated rate constant for the adsorption of Mo onto Fe oxyhydroxides is 36 ± 45 cm3 mol−1 s−1.Diagenetic modeling indicates that authigenic Mo in sediments of the perennially oxic basin represents about one-third of the total solid phase Mo in the first cm below the sediment-water interface and only one tenth below this horizon. If we assume that no authigenic Mo is accumulated in the seasonally anoxic lake sediments we conclude that the sediment Mo concentrations, which are up to 3-16 times higher than the average lithogenic composition, depending on the lake, are mainly due to atmospheric deposition of anthropogenic Mo and not to the formation of authigenic Mo phases. Reconstructed historical records of the atmospheric Mo deposition indicate maximum values in the 1970s and 1980s and significant decreases since then. Emissions to the atmosphere associated with the smelting of non-ferrous ores and coal combustion appear to be the most important sources of anthropogenic Mo.  相似文献   

12.
Concentrations of Re and Os, and the isotopic composition of Os have been measured in the Japan Sea sediments to assess the response of the Japan Sea to glacial–interglacial climate change and associated weathering fluxes. The osmium concentrations in the sediment samples analyzed vary from 59 to 371 pg/g, and 187Os/188Os from 0.935 to 1.042. Only 187Os/188Os of sediment samples from dark laminations deposited under suboxic to anoxic conditions and having elevated concentrations of Re and Os, and with ≥ 80% hydrogenous Os are explained in terms of seawater composition. Lower 187Os/188Os were observed for sediments deposited during the last glacial maximum (LGM) when planktonic foraminifera from the Japan Sea recorded lighter oxygen isotopic composition. Decrease in dissolved Os fluxes from continents and/or change in the composition of the dissolved load to the Japan Sea are suggested as the driving mechanisms for the observed lower LGM 187Os/188Os. The results of this study, coupled with lower 187Os/188Os during the last glacial observed at other sites from ocean basins with different lithology and contrasting sediment accumulation rates, suggest that this trend is characteristic of the global oceans.

Data from this study show that the Japan Sea recorded higher 187Os/188Os during the current interglacial coinciding with excursions of oxygen isotopic compositions of planktonic foraminifera to heavier values. This is explained in terms of preferential release of 187Os during deglacial weathering and/or higher continental Os flux driven by warm and wet climate. This study demonstrates that Os isotopic composition of reducing margin sediments has immense potential to track variations in the seawater composition. In addition, 187Os/188Os of reducing sediments may be used to draw inferences about local paleoceanographic processes in semi-enclosed basins such as the Japan Sea.  相似文献   


13.
Concentrations and fluxes of particulate U were measured throughout the water column at several locations in the Atlantic and Pacific Oceans with in situ filtration systems and sediment traps. The results indicate that dissolved U is fixed to particles in surface seawater. Organic matter appears to be the carrier phase. Formation of particulate authigenic U below the surface waters could not be detected. Authigenic U is remineralized within the bathypelagic layers at the open ocean sites studied. In the Panama Basin, an upwelling area with high biological productivity, remineralization of authigenic U in the deep water column was not observed. The rate of remineralization of authigenic U in the deep sea is insufficient to produce a measurable concentration gradient between surface and deep waters within the mixing time of the oceans. Formation of authigenic U in the water column in areas such as the Panama Basin is not a significant sink for U on an ocean wide basis.  相似文献   

14.
通过对南海北部的ODP 1148站岩芯600 mcd以上(约30 Ma以来)的沉积物中自生富集Mn、Cd和Mo等过渡金属元素的含量变化的研究,并结合相关的化学组成结果,探讨了岩芯内部氧化-还原条件的变化以及相关元素的活动特征,反演了相应沉积时期的环境演变.结果显示,岩芯387 mcd以上,自生Mn富集明显,代表氧化的环境;387～485 mcd之间,自生Cd含量明显富集,Mn含量显著降低,代表少氧的环境;485 mcd以下,Mn和Cd含量极低,自生Mo明显富集,代表缺氧的环境.随氧化-还原条件的变化,Mo存在明显的向下迁移并在缺氧界面的缺氧一方达到最大值的趋势,而Cd在少氧环境形成的固相态则可能在缺氧环境下不稳定,溶解态的Cd有向上迁移的趋势,并且在少氧/缺氧界面的少氧一方富集.这些过渡金属元素记录的氧化-还原条件的变化,反映出ODP 1148站所在海区的沉积环境变化:早期有较丰富的陆源输入,表层海水生产力较高,随着南海不断扩张以及全球海平面上升,该海区表层海水生产力逐渐降低.  相似文献   

15.
In ocean margin sediments both marine and terrestrial organic matter (OM) are buried but the factors governing their relative preservation and degradation are not well understood. In this study, we analysed the degree of preservation of marine isoprenoidal and soil-derived branched glycerol dialkyl glycerol tetraethers (GDGTs) upon long-term oxygen exposure in OM-rich turbidites from the Madeira Abyssal Plain by analyzing GDGT concentrations across oxidation fronts. Relative to the anoxic part of the turbidites ca. 7-20% of the soil-derived branched GDGTs were preserved in the oxidized part while only 0.2-3% of the marine isoprenoid GDGT crenarchaeol was preserved. Due to these different preservation factors the Branched Isoprenoid Tetraether (BIT) index, a ratio between crenarchaeol and the major branched GDGTs that is used as a tracer for soil-derived organic matter, substantially increases from 0.02 to 0.4. Split Flow Thin Cell (SPLITT) separation of turbidite sediments showed that the enhanced preservation of soil-derived carbon was a general phenomenon across the fine particle size ranges (<38 μm). Calculations reveal that, despite their relatively similar chemical structures, degradation rates of crenarchaeol are 2-fold higher than those of soil-derived branched GDGTs, suggesting preferential soil OM preservation possibly due to matrix protection.  相似文献   

16.
Nine natural decay-series isotopes were measured in six box cores collected from a transect across the Santa Monica Basin. The 210Pb-derived sedimentation rate decreases from ~80 mg/cm2-yr at the slope to ~20 mg/cm2-yr in the deep central basin. Sediment mixing prevails in sites underlying oxic waters, but is subdued in the anoxic deep basin below the sill depth. Uranium contents in sediments are controlled by levels of authigenic U, which are higher in the more reduced condition in the deep basin. Most of the authigenic U results from precipitation within the sediments.The 232Th-228Th disequilibrium in sediments indicates that 228Ra is lost from the sediments from a depth of ~ 10 cm upward. Modelling the distribution of excess 228Th and 234Th in the surficial layers of the deep basin sediments results in a mean sediment mixing coefficient of 0.2 cm2/yr and a sedimentation rate close to that based on 210Pb. There is no evidence of changing sedimentation rate in the central basin during the past century. Fluxes of excess 210Pb, 230Th and 231Pa to the central Santa Monica Basin sediments are much higher than what can be predicted from local supply. Advective input of open ocean waters coupled with enhanced scavenging of these reactive nuclides at the ocean margin is considered to be the primary cause.  相似文献   

17.
In this study, redox-dependent phosphorus (P) recycling and burial at 6 sites in the Baltic Sea is investigated using a combination of porewater and sediment analyses and sediment age dating (210Pb and 137Cs). We focus on sites in the Kattegat, Danish Straits and Baltic Proper where present-day bottom water redox conditions range from fully oxygenated and seasonally hypoxic to almost permanently anoxic and sulfidic. Strong surface enrichments of Fe-oxide bound P are observed at oxic and seasonally hypoxic sites but not in the anoxic basins. Reductive dissolution of Fe-oxides and release of the associated P supports higher sediment-water exchange of PO4 at hypoxic sites (up to ∼800 μmol P m−2 d−1) than in the anoxic basins. This confirms that Fe-bound P in surface sediments in the Baltic acts as a major internal source of P during seasonal hypoxia, as suggested previously from water column studies. Most burial of P takes place as organic P. We find no evidence for significant authigenic Ca-P formation or biogenic Ca-P burial. The lack of major inorganic P burial sinks makes the Baltic Sea very sensitive to the feedback loop between increased hypoxia, enhanced regeneration of P and increased primary productivity. Historical records of bottom water oxygen at two sites (Bornholm, Northern Gotland) show a decline over the past century and are accompanied by a rise in values for typical sediment proxies for anoxia (total sulfur, molybdenum and organic C/P ratios). While sediment reactive P concentrations in anoxic basins are equal to or higher than at oxic sites, burial rates of P at hypoxic and anoxic sites are up to 20 times lower because of lower sedimentation rates. Nevertheless, burial of reactive P in both hypoxic and anoxic areas is significant because of their large surface area and should be accounted for in budgets and models for the Baltic Sea.  相似文献   

18.
The concentrations of authigenic phases of Cd, Re, U, and Mo increase with depth in four 45-cm-long sediment box cores collected along the axis of the Laurentian Trough, Gulf of St. Lawrence. Average authigenic accumulation rates, estimated from element inventories, are similar to rates in other continental margin environments. Strong regional variations in sediment accumulation rate and sulfide concentration have little influence on the accumulation rates of Cd and Re. This suggests that slow precipitation kinetics controls the accumulation of Cd and Re in these sediments. The accumulation rate of authigenic U is more variable; it may be tied to the kinetics of microbially mediated U reduction and be controlled by the availability of reactive organic matter. Authigenic Mo is distinguished by a sharp subsurface concentration minimum, above which Mo cycles with manganese. Mo released to pore water upon reduction of Mn oxides diffuses downward and enriches the subsurface sediment. Mo accumulates most rapidly in the sediment with the highest sulfide content. Slow conversion of molybdate to thiomolybdate may explain the much slower Mo accumulation rate in the less sulfidic sediments. A component of authigenic Mo accumulates with pyrite in an approximately constant Mo:Fe ratio. The accumulation rate of pyrite and associated Mo is insensitive to AVS abundance. Pyrite formation may be limited by the reactivity of iron oxide minerals.  相似文献   

19.
We measured U in sediments (both pore waters and solid phase) from three locations on the middle Atlantic Bight (MAB) from the eastern margin of the United States: a northern location on the continental shelf off Massachusetts (OC426, 75 m water depth), and two southern locations off North Carolina (EN433-1, 647 m water depth and EN433-2, 2648 m water depth). These sediments underlie high oxygen bottom waters (250-270 μM), but become reducing below the sediment-water interface due to the relatively high organic carbon oxidation rates in sediments (EN433-1: 212 μmol C/cm2/y; OC426: 120 ± 10 μmol C/cm2/y; EN433-2: 33 μmol C/cm2/y). Pore water oxygen goes to zero by 1.4-1.5 cm at EN433-1 and OC426 and slightly deeper oxygen penetration depths were measured at EN433-2 (∼4 cm).All of the pore water profiles show removal of U from pore waters. Calculated pore water fluxes are greatest at EN433-1 (0.66 ± 0.08 nmol/cm2/y) and less at EN433-2 and OC426 (0.24 ± 0.05 and 0.13 ± 0.05 nmol/cm2/y, respectively). Solid phase profiles show authigenic U enrichment in sediments from all three locations. The average authigenic U concentrations are greater at EN433-1 and OC426 (5.8 ± 0.7 nmol/g and 5.4 ± 0.2 nmol/g, respectively) relative to EN433-2 (4.1 ± 0.8 nmol/g). This progression is consistent with their relative ordering of ‘reduction intensity’, with greatest reducing conditions in sediments from EN433-1, less at OC426 and least at EN433-2. The authigenic U accumulation rate is largest at EN433-1 (0.47 ± 0.05 nmol/cm2/y), but the average among the three sites on the MAB is ∼0.2 nmol/cm2/y. Pore water profiles suggest diffusive fluxes across the sediment-water interface that are 1.4-1.7 times greater than authigenic accumulation rates at EN433-1 and EN433-2. These differences are consistent with oxidation and loss of U from the solid phase via irrigation and/or bioturbation, which may compromise the sequestration of U in continental margin sediments that underlie bottom waters with high oxygen concentrations.Previous literature compilations that include data exclusively from locations where [O2]bw < 150 μM suggest compelling correlations between authigenic U accumulation and organic carbon flux to sediments or organic carbon burial rate. Sediments that underlie waters with high [O2]bw have lower authigenic U accumulation rates than would be predicted from relationships developed from results that include locations where [O2]bw < 150 μM.  相似文献   

20.
The concentration of mercury in contaminated estuarine sediments of Bellingham Bay, Washington was found to decrease with a half-time of about 1.3 yr after the primary anthropogenic source of mercury was removed. In situ measurements of the mercury flux from sediments, in both dissolved and volatile forms, could not account for this decrease. This result suggests that the removal of mercury is associated with sediment particles transported out of the study area. This decrease was modeled using a steady-state mixing model.Mercury concentrations in anoxic interstitial waters reached 3.5 μg/l, 126 times higher than observed in the overlying seawater. Mercury fluxes from these sediments ranged from 1.2 to 2.8 × 10?5 ng/cm2/sec, all in a soluble form. In general, higher Hg fluxes were associated with low oxygen or reducing conditions in the overlying seawater. In contrast, no flux was measurable from oxidizing interstitial water having mercury concentrations of 0.01-0.06 μ/l.  相似文献   

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